The present invention relates to a method for maintaining the heat in an exhaust aftertreatment system connected to the exhaust pipe of an internal combustion engine, which engine is used to propel a vehicle and is equipped with a valve-controlled EGR duct for adjustable recirculation of exhaust gases from the exhaust side of the engine to its induction side.
In internal combustion engines with exhaust aftertreatment systems, it is desirable for these aftertreatment systems to be able to operate within a favorable temperature range, for example 250-350 degrees Celsius. Sometimes, the operating conditions of an internal combustion engine can be such that the exhaust gas temperature is too low for said temperature range to be able to be maintained. One example of such operating conditions is when the engine is dragging, i.e. a vehicle which is normally driven by the engine is coasting down a hill. In this instance, the engine is in principle pumping fresh air to the exhaust system.
It is known to supply hydrocarbons to the exhaust gas stream in order to increase the temperature and thereby maintain a temperature range. The drawback with such methods is increased fuel consumption. If the temperature of the exhaust gas stream is low, more hydrocarbons are needed in order to maintain the temperature range. Increased emission control requirements have therefore often resulted in a loss of efficiency of the internal combustion engine. It is therefore important to produce methods which allow effective exhaust emission control without adversely affecting the efficiency of the engine.
Recirculation of exhaust gases, so-called EGR (Exhaust Gas Recirculation), is a known method in which a part of the total exhaust gas flow of the engine is recirculated and this subflow is fed to the inlet side of the engine, where it is mixed with incoming air for introduction into the cylinders of the engine. It thereby becomes possible to reduce the quantity of nitrogen oxide in the exhaust gases.
It is desirable to produce a method of using an internal combustion engine with an exhaust gas recirculation system, which method makes it possible to maintain a temperature range in the exhaust gas stream without unnecessary adverse effect upon the efficiency of the engine.
A method according to an aspect of the invention is characterized by the steps of detecting that neither the braking system nor the throttle control mechanism of the vehicle is activated and that the vehicle is being driven at a speed in excess of a predetermined value, and of regulating the gas flow through the EGR duct so that the gas flow to the exhaust aftertreatment system is reduced to a level which is less than about 50% of the gas flow to the exhaust aftertreatment system with shut-off EGR valve. By virtue of this method, heat losses in the exhaust gas stream to an exhaust aftertreatment system are prevented under certain driving conditions.